One of the old alternative photographic methods I became interested is Cyanotype or blueprint.  This technique uses light sensitivity for ferrous salts when results are blue images. The Cyanotype was discovered by English scientist Sir John Herschel in 1842.

In a typical procedure, equal volumes of an 8.1% (w/v) solution of potassium ferricyanide and a 20% solution of ferric ammonium citrate are mixed. This mildly photosensitive solution is then applied to a receptive surface (such as paper or cloth) and allowed to dry in a dark place. Cyanotypes can be printed on any surface capable of soaking up the iron solution. Although watercolor paper is a preferred medium, cotton, wool and even gelatin sizing on nonporous surfaces have been used. Care should be taken to avoid alkaline-buffered papers, which degrade the image over time.

A positive image can be produced by exposing it to a source of ultraviolet light (such as sunlight) through a contact negative (which can be produced by any suitable means, e.g. a conventional photographic negative or a print on acetate film made using photo-processing software to invert a positive monochrome digital image). The UV light reduces the iron(III) to iron(II). This is followed by a complex reaction of the iron(II) complex with ferricyanide. The result is an insoluble, blue dye (ferric ferrocyanide) known as Prussian blue.

Exposure to ultraviolet light reduces the iron in the exposed, turning the paper a steel-grey-blue color. The extent of color change depends on the amount of UV light, but acceptable results are usually obtained after 10-20 minute exposures on a dark, gloomy day. The highlight values should appear overexposed, as the water wash reduces the final print values. Prints can be made with large format negatives and lithography film, or everyday objects can be used to make photograms.

After exposure, developing of the picture involves the yellow unreacted iron solution being rinsed off with running water. Although the blue color darkens upon drying, the effect can be accelerated by soaking the print in a 6% (v/v) solution of 3% (household) hydrogen peroxide. The water-soluble iron(III) salts are washed away, while the non-water-soluble Prussian blue remains in the paper. This is what gives the picture its typical blue color.

The overall contrast of the sensitizer solution can be increased with the addition of approximately 6 drops of 1% (w/v) solution potassium dichromate for every 2 ml of sensitizer solution.

Cyanotype images can be toning by tanin (tea, cofee, red wine)

Source: Wikipedia

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